1. Field of the Invention
The invention relates to the process and construction of wastewater treatment plants. More specifically, the invention relates to a system for removing contaminants from wastewater.
2. Background Art
Municipalities are often faced with the challenge of removing polychlorinated biphenyl (PCB) and other organic compounds from the discharge of their aging wastewater treatment facilities. Polychlorinated biphenyl is a man-made compound that was used in the manufacture of transformers, hydraulic oils, paints, and in other products. It has been determined to be a health risk to humans and is seen as a bio-accumulative chemical of concern. The PCB, which may be found in the plant's influent and sometimes in the effluent, must be removed to meet a limitation on amounts that are less than can be detected. New facilities may be required to provide the best available treatment for PCB's.
Past manufacturing and disposal procedures have caused the dispersal of PCB in various areas. One of the properties of PCB which made it an attractive product was its resistance to environmental breakdown. Medical research discovered that small concentrations of PCB could cause birth defects and consequently, the manufacture of PCB has been prohibited.
A municipal sewer system may be a combined sewer system subject to periodic flooding. This may spread PCB contamination throughout the sewer system. With PCB's affinity to solids, PCB may be adhered to the sewer walls and solids within the sewer system, and the PCB will periodically be released into the sewage for many years to come.
However, this environmental persistence of PCB prompted the EPA to regulate its discharge from wastewater treatment plants. Because PCB is potentially harmful even at low concentrations, the EPA has set the discharge limitation for PCB at 0.02 parts per trillion, which is 5000 times lower than concentrations (0.1 parts per billion) which current laboratory technology can detect.
It is known that PCB has a large molecular weight and that it is not readily soluble in water. However, it is highly adsorbed by activated carbon. Activated carbon is a granular charcoal that is made by the process of grinding and burning bituminous coal. The size of a carbon grain is about the size of sand (about 0.8 to 1.0 mm in diameter). When water passes through a filter bed of granular activated carbon, the carbon removes dissolved organic contaminants from water by adsorption. During the adsorption process, organic molecules diffuse into the pores of the carbon granules and physically or chemically attach to the carbon. But eventually, a carbon bed becomes saturated with PCB or the concentration exceeds an acceptable limit. At that time, the carbon must be replaced or an alternative solution is needed.
Several authorities have voiced concerns regarding the combined effects of bio-growth and oil accumulation at treatment plants that lack sand pre-filtration facilities.